This invention relates to a key telephone apparatus and, more particularly, to an attachment for allowing an intercom station in a key telephone system to obtain access to a trunk telephone line.
Key telephone systems serve to interconnect telephone substations located on a private premises with the telephone lines or trunk lines, as variously termed, supplied to the premises by the local telephone company, as well as to provide an "on-premises" communication system. Typically the key telephone system includes some substations that may access the trunk lines directly and also includes a plurality of substations, which are denoted "intercom substations", for providing communications between locations on the premises. In common key telephone systems, one or more telephone intercom substations are connected via two intercom lines to two common intercom circuits in the key service unit so that any party desiring may pick up the handset at an intercom telephone substation, thereby going "off-hook" and place the substation on the intercom line circuit. In the event that the one intercom circuit is in use, the party would intercept any conversation currently in progress and cannot use that intercom line. The party can then position a selector "push key" provided on the telephone instrument to the position of the second intercom line circuit and access the second intercom line. The user can then use the intercom line to reach another intercom substation. Each intercom substation includes a sounder which may be selectively actuated over a control line either by the operator controlling the key service unit, or by another intercom substation if the party at such substation dials the digits of the intercom substation wherein the key service unit decodes the dialed digit and applies the control signal to the sounder of the called substation.
Customarily the intercom stations do not have access to the trunk telephone lines. Hence while persons having access to the intercom substation are permitted to place a call from one intercom station to another to carry on an on-premises conversation, such persons cannot use the intercom substations for placing calls off the premises. Likewise calls coming in over the trunk lines cannot be connected to the intercom substation. Thus when an incoming outside call is received over an incoming trunk line at the operator's key service unit, the operator ascertains the identity of the person whom the calling party desires to reach. If that person is located at an intercom substation, the operator dials the extension number of that intercom substation, and the key service unit of the system operates its switching devices to select a line relay associated with the units and 10 digits of the called intercom substation. When the line relay is actuated an electrical circuit is closed over the control line to the sounder contained in the called intercom substation. The called party in answer picks up the handset and receive the information from the operator and if the called party desires to speak directly to the calling party, the called party must call back over a regular substation. The called party cannot reach the calling party over the intercom line. These key telephone systems are of a conventional and well known structure. Typical of key telephone systems in commercial use are the 1A22model manufactured by ITT Company, and the Litkey key telephone system distributed by Litton BTS, and the T.I.E. Company key telephone system. Conceivably there are circumstances where it is desirable to allow a telephone intercom substation to directly access a trunk line in order to avoid the aforedescribed inconvenience and to obtain a greater flexibility for an existing key telephone system.
In my prior application for patent, Ser. No. 787,361, filed June 16, 1975, of which this application is a division, a unique apparatus for a key telephone system for enabling intercom stations to access trunk lines is described.
Briefly the intercom to trunk access unit of that invention includes a trunk selector which may selectively connect to a plurality of trunk telephone lines entering the key telephone system, a selector advance unit connected to the trunk selector for advancing the selector in response to input pulses, such as dial pulses. A busy detector is included in the trunk selector and an advance-on-busy unit, which has an input connected to the busy detector and has an output connected to the selector advance unit, to supply a pulse to the selector advance unit in response to an output from the busy detector whereby the trunk selector is automatically advanced to the next trunk line. As a further aspect a busy-defeat unit for preventing selector advance-on-busy is connected to and controlled jointly by the key service unit and by the intercom substation to couple an output to the advance-on-busy unit. Relay means, located at the key service unit remote from the telephone substation, is coupled to the intercom substation for coupling the intercom station to the trunk access unit. And push button switch means are located at the intercom substation for actuating the relay means.
In the operation of that unit, the removal of the substation handset places the intercom substation in the "off-hook" condition. Operation of the push button switch at the intercom station actuates the relay means to couple the intercom station through the the trunk access unit. The trunk access unit awaits and responds to a dialed digit, such as "1", which actuates the selector advance and that in turn steps the trunk selector to the corresponding first trunk position. If the trunk is not in a busy condition the selector connects the intercom communication line through to the first trunk line. However if the first selected line is in a busy condition, the busy detector signals the advance-on-busy circuit which in turn supplies a control signal or pulse to the selector advance unit and in response the advance steps the trunk selector to the next trunk and the next trunk is in turn tested for "busy". This process of "trunk hunting" continues until the first available non-busy trunk line is reached, whereupon the intercom line is connected through to that trunk line.
Further in accordance with that invention, if the intercom station is placed in an off-hook condition in response to a signal from the operator, the busy-defeat unit receives one input, such as from the line relay associated with the called substation. By remaining off-hook, the party simply operates the push button switch at the intercom station and accesses the intercom-to-trunk access unit. In so doing, the push button switch further supplies the second input to the busy-defeat circuit, which thereupon disables the busy-advance unit. The party at the substation dials the digit of the selected trunk. The digit pulses are received by the selector advance unit which steps the trunk selector from the initial position over to the position of the selected trunk line and the selector switches through the intercom station to the trunk line. Inasmuch as the party is responding to a call on the selected trunk line, the trunk line would otherwise show busy and prevent the selector from stopping at that trunk hence the busy-defeat circuit disables the advance-on-busy unit.
It is recognized that the aforedescribed apparatus permits use of a single communications circuit by each of the individual telephone subsets. To ensure privacy of any call established between an intercom substation and an outside trunk line over that communications circuit and to ensure proper operation of the circuits limiting the access to the aforedescribed line to only one intercom subset at any time is desired.